Severe acute respiratory syndrome (SARS) is a newly emerged global disease of significant economic impact and bioterrorism potential. It is caused by a new strain of coronavirus (SARS CoV) genetically only distantly related to CoV groups I-III. However phylogenetic analysis of SARS CoV suggests distant relationships to CoV group II or an evolutionary origin through recombination between mammalian (group I) and avian (group III) CoVs. Recent data suggests that SARS is zoonotic and may have a broad host range. Genetically similar SARS CoV were isolated from civet cats and raccoon dogs in China and from a recent human SARS case in China suggesting the latter SARS virus reemerged from an animal reservoir. SARS CoV experimentally infects macaques, ferrets and cats. Antigenic cross-reactivity, likely related to antibodies to other CoVs, is observed in ELISA tests for SARS CoV antibodies. This cross-reactivity compromised rapid diagnosis of the recent SARS cases in China and the suspect 2003 summer cases in Canada (likely OC43), hindering early detection needed to prevent SARS spread and new epidemics. It further hinders serologic screening of SARS antibodies in animal and human populations and identification of current or emerging (rats, cats?) animal reservoirs for SARS. Based on our preliminary data and the findings of others (Ksiazek et al 2003), such cross-reactivity is most commonly seen between SARS CoV and group I CoVs (N protein) but the biologic basis is unknown. Our goal is to investigate the antigenic basis for the two-way crossreactivity between SARS CoV and animal CoVs, to develop SARS CoV antibody specific ELISA tests and to apply these to sera from domestic and wild animals to identify potential animal reservoirs for SARS. Specifically we will: 1) Assess the two-way antigenic cross-reactivity of SARS CoV with animal group I, II and III CoVs in serologic assays including ELISA, immunofluorescence (IF) and Western blots. 2) Determine the CoV proteins involved in the two-way antigenic cross-reactivity between animal CoVs and SARS CoV using both SARS and animal CoV recombinant N and S proteins expressed in a baculovirus system. 3) Evaluate the region of the CoV protein(s) responsible for the cross-reactivity using N and S fragments and truncate this region to eliminate cross-reactivity in serologic tests to make SARS CoV specific assays. 4) Test sera from experimentally inoculated and field exposed animals for antibodies cross-reactive with SARS CoV using the intact and truncated SARS CoV S and N proteins: a) As an innovative approach test serum from gnotobiotic pigs recovered from TGEV and boosted with inactivated SARS CoV to assess in vivo crossreactivity; b) Test serum from domestic livestock, poultry and wildlife (wild ruminants and carnivores). Our past 30 years of knowledge and experience working with animal CoVs, our extensive collections of multiple strains of animal CoVs from diverse species including wildlife and the corresponding hyperimmune antisera and monoclonal antibodies, our TGEV CoV S and N gene clones and appointment of our lab by WHO as an animal CoV SARS network lab will provide the expertise and reagents needed to thoroughly investigate the antigenic interrelationships among animal and human CoVs including SARS CoV.